This invention relates to control circuits for electrically driven metallographic polishing apparatus.
In polishing apparatus of the type in which a circular disc is employed with a relatively fine grit abrasive for polishing metallographic specimens for microscopic examination, great care must be taken to prevent damage to the specimens being polished since they frequently are of considerable value andd irreplaceable. Should, for example, the polishing speed increase significantly beyond the selected speed, the specimen being polished can be seriously damaged or even destroyed. With many prior art polishers, the speed is manually selected and can vary significantly with mechanical load and line voltage variations. Although closed loop feedback speed control systems are generally known, the use of a simple feedback system could, should a component of the control systems fail, cause the motor to uncontrollably increase its speed thereby damaging or destroying the specimen. Although speed control to provide a constant polishing speed under varying loads is desirable, heretofore, conventional control systems have not successfully reduced the possibility of sample damage or destruction.